1. Field of the Invention
The present invention relates to an ignition device for a multi-cylinder internal combustion engine.
2. Description of the Prior Art
In order to obtain a good drivability and obtain a desired fuel performance, it is necessary to match exactly the ignition timing for cylinder positions of an engine. If the ignition timing in the cylinder can adapt perfectly during all kinds of driving conditions, then the earliest igniting position will be approximately 60 degrees before top dead center and the last igniting position will be approximately 40 degrees after top dead center in the rotary angle of a crankshaft. Accordingly a distributor must secure the electrical connection between the center electrode and one of the side electrodes within 100 degrees in the rotary angle of the crankshaft i.e. 50 degrees in the rotary angle of a distributor shaft or cam shaft.
When an internal combustion engine has six cylinders, the six side electrodes are required to be equally spaced from each other and the disconnecting intervals between the rotor and each side electrode is only 10 degrees (360/6-50=10) in the angle of the distributor shaft.
When an internal combustion engine has eight cylinders, the disconnecting interval does not exist. In this case the possibility of simultaneous connection across two side electrodes by the rotor arises, and if a high voltage for a spark is applied to the rotor in this case, the high voltage does not determine the supply to either side electrodes. Accordingly the cylinder generating the spark for ignition is not determined alternatively, thus the internal combustion engine cannot operate smoothly. In the case of six cylinders, when the pitch circles of the side electrodes are not large in diameter, the intervals corresponding to the angle of 10 degrees of the distributor shaft is not so large and also the cylinder generating the spark is not determined. The above mentioned spark which is not distributed appropriately and is generated at undesirable positions, and is called a flashover. In order to prevent flashover, in the above mentioned circumstances, the pitch circles of the side electrodes i.e. the outer diameter of the distributor, must be either made larger or the non optimum ignition timing will undesirably continue.
According to the above mentioned reasons, methods by which an engine has two distributors and each distributor supplied a high voltage to half the cylinders, respectively, and by which basically two distributors have a common rotor shaft and are unified, are being experimentally examined. However, the method employing two distributors has defects requiring double the production cost and requiring a wider space. Although the method using a common rotor shaft has a spatial advantage the method needs two ignition coils, thus the method has the disadvantage of a high production cost. Moreover, the incorporating of the two distributors into the construction becomes a complicated structure, thus because of the fault flashover is generated easily and frequently.
The present invention has been proposed in order to solve the above mentioned problems inherent in the conventional methods.
The above mentioned conventional ignition device has been disclosed in Japanese Patent Application No. 85036/74 (Japanese Patent Laid-Open No. 43327/75).